We are investigating the origin and the significance of changes in the nucleosomal proteins during different stages of development, proliferation and terminal differentiation of different cell types of the mouse, by studying a) the interactions of nonallelic primary structure histone variants and minor species with other histones to form high affinity complexes, b) the distribution of the various histone components in chromatin regions of different functional states, c) the changes in histone metabolism which lead to drastic changes in the relative amounts of different sets of histone viriants during terminal differentiation of a variety of cells, d) the relationship between changes in the nucleosome proteins and the proliferation and differentiation of normal and transformed hepatocytes and Friend erythroleukemia cells before and after induction of terminal differentiation.